New docker release deprecates boot2docker and favors new Docker Machine

[jacks205]

https://docs.docker.com/installation/mac/

[donnemartin]

@jacks205 thanks for the reminder. I've recently migrated to docker machine already but still need to update the repo.

[jpwalsh1]

Update Docker install lines in brew.sh to the following to install current Docker.

brew cask install --appdir="/Applications" docker